The use of horizontal wells in oil reservoirs is currently of high interest within the oil industry. Horizontal wells allow more reservoir surface area to be contacted and thereby reduce inflow pressure gradients for reasonable oil production rates. Alternatively, for typical pressure gradients within the wellbore region, the productivity of a horizontal well is greater than that in a vertical well.
Several possible benefits of horizontal wells are currently being exploited in the Canadian tar sands. Reservoirs in Canada that may be categorized as immobile under reservoir conditions include the Cold Lake and Athabasca deposits. Horizontal wells are also being used to produce mobile viscous oil from a formation.
Current practices for producing the above immobile tar sands and mobile viscous oils include mining and steam stimulation by formation fracturing. However, mining is not practical below very shallow depths. Furthermore, steam stimulation by formation fracturing is not feasible in those reservoirs underlain by water aquifers. In general, fracturing in zones underlain by water aquifers or a bottom water zone results in large amounts of water production and non-uniform development of the steam zone. Large water influx is due to penetration of the fracture into the water aquifer and water coning around the wellbore.
Water coning is the phenomenon whereby water is drawn upwardly from the water-bearing portion into the oil-bearing portion about the well.
Steam stimulation below fracture pressure in a vertical well is not practical due to the very low injectivity of the formation to steam and the very small area of reservoir contact. Increased area of contact can be achieved by the use long horizontal wells (1,000 to 3,000 feet as compared to 30 to 100 feet for a vertical well). This increased area of contact allows more of the reservoir's area to be heated by steam injection. This results in more oil production due to increased volume of the heated zone. Injection of a large steam slug into a horizontal well underlain by a water aquifer may result in a fracture into the aquifer.
The length of horizontal wells permits smaller inflow pressure gradients during isothermal viscous oil production. However, the shape of the pressure profile around the wellbore remains logarithmic such that the largest pressure gradients occur in a near-wellbore region. Hence, water coning, while reduced in horizontal wells compared to vertical wells is still quite a problem in viscous oil reservoirs, since a sharp pressure sink exists in the near-wellbore region.
Therefore, what is needed is a method to reduce water coning in a horizontal well so as to obtain an increased production of hydrocarbonaceous fluids from a heavy oil reservoir.